The present invention relates to an apparatus and method for controlling pressure surges in pipelines, and more particularly to an apparatus and method for controlling pipeline pressure surges by responding to pressure surges to limit its overshoot or undershoot to less than 10 psi (7 N/cm.sup.2) for a surge magnitude of 40 psi/sec (28 N/cm.sup.2 /sec).
Pipelines for transporting crude oil from wells to refineries and refined products from refineries to distribution points require pumping stations to be connected at various points along the pipeline. Since pipeline pressure gradually decreases with distance along the pipeline due to friction of the internal walls of the pipeline, pumping stations must be used to periodically boost the pipeline pressure so that a designed flow rate through the pipeline is maintained.
Pressure surges are created within the pipeline as pumps in the pumping stations are turned on and off and as valves are opened and closed. The shut down of a pump station causes a positive pressure surge of increased pressure that travels within the pipeline in the opposite direction of liquid flow, or upstream, and a negative pressure surge of decreased pressure that travels in the direction of flow, or downstream. As noted by H. A. Brainerd in "Good Surge Control Can Help Pipeline Throughput," pressure surges travel through the liquid in a pipeline at sonic velocity, which typically varies from 3,000-4,000 feet per second, depending upon the nature of the liquid and the physical characteristics of the pipeline. The magnitude of such a pressure surge may be as small as one pound per square inch (psi) per second (0.7 Newtons per square centimeter (N/cm.sup.2) per second) or greater than 2,000 psi per second (1380 N/cm.sup.2 per second). The magnitude and velocity of a pressure surge decrease as the surge travels through the pipeline, in part due to the stretch of the pipeline and the compressibility of the liquid.
Since a significant portion of the cost of a pipeline system is the piping itself, it is economically advantageous to use pipes having the thinnest walls as possible. However, the incentive to use thin-walled pipes must be balanced with the possibility that the pipeline might burst due to an uncontrolled positive pressure surge, thus causing a significant economic burden due to product loss and environmental damage. Since pipelines are generally operated at 75% to 90% of the pipe yield strength, with 500 to 2,000 psi (345 to 1380 N/cm.sup.2) being the range of typical operating pressures, a pipeline rupture could be caused by an overpressure on the order of a mere 50 psi (35 N/cm.sup.2).
In addition to the pipeline rupture problem caused by positive pressure surges, negative pressure surges are undesirable because they may damage the pump motors due to cavitation. Cavitation, which is a well known problem, may occur when the upstream pipeline pressure falls below the vapor pressure of the fluid. Cavitation may cause damage to pump chamber walls, impellers, and other pumping station surfaces which come into contact with the fluid. Thus, it is desirable to control negative pressure surges upstream of the pumping station.
Typical pipeline systems include one or more controllers for controlling the pipeline pressure at each pumping station. Where multiple controllers are used, one controller is typically used to control the downstream, or discharge pressure, and a separate controller is used to control the upstream, or suction pressure. A third controller may be used to control the motor current of a motor used to drive the pump used in the pumping station.
The outputs of the controller or controllers are typically used to control a valve operatively connected to vary the pipeline flow on the downstream side of the pumping station. The pipeline pressure is controlled by opening or closing the valve. Opening the valve causes the suction pressure to be decreased and the discharge pressure to be increased, and closing the valve causes the suction pressure to be increased and the discharge pressure to be decreased. Because it reduces the amount of energy needed to maintain a predetermined flow, the valve is typically held wide open, and is only partially closed when either the suction pressure drops below a predetermined suction pressure setpoint or when the discharge pressure rises above a predetermined discharge setpoint.
A conventional pipeline system typically includes a high pressure sensing system which responds to any discharge pressure above a first, relatively high, predetermined shutdown pressure by shutting down the motor in the pumping station in order to prevent pipeline rupture. A low pressure sensing system is also provided to shut down the pipeline in case the suction pressure drops below a second, relatively low, predetermined shutdown pressure to prevent cavitation damage.
In a typical pipeline system, the discharge (pressure is controlled with respect to a discharge pressure setpoint, which is typically 50-100 psi (35-70 N/cm.sup.2) below the discharge shutdown pressure. The magnitude of the pressure difference between the discharge setpoint and discharge shutdown pressure is a measure of the effectiveness of the pressure control system. It is desirable to keep this pressure difference at a minimum. Since the discharge shutdown pressure depends upon the yield strength of the pipeline, the shutdown pressure is relatively fixed. If the discharge pressure setpoint can be made higher, or closer to the discharge shutdown pressure, the flow through the pipeline will increase. Thus, as the pressure difference between the discharge pressure setpoint and the discharge shutdown pressure is minimized, the flow through the pipeline is maximized, thus achieving the maximal economic benefit.
An ideal pipeline pressure control system would be able to control the actual discharge pressure to within 1 psi, for example, of the discharge pressure setpoint, so that the setpoint could be set just below the shutdown pressure to maximize flow. However, pipeline pressure control systems are not ideal and, as indicated above, the discharge pressure setpoint is typically set 50-100 psi (35-70 N/cm.sup.2) below the shutdown pressure.
Prior pipeline pressure control systems have inherent disadvantages which limit their effectiveness in quickly responding to pressure surges. For example, conventional pressure control systems typically use proportional/integral/derivative (PID) control modes. PID control is a well known, conventional type of control in which the actual pipeline pressure is repeatedly sampled in a digital system and compared to a predetermined pressure setpoint. After each comparison, the position of the valve is adjusted based upon the sign and magnitude of the pressure difference. Variations of PID control are also used, such as proportional-only control and proportional and integral (PI) control. Conventional PID controllers have the capability to perform all three types of control: proportional-only control, PI control, and PID control.
During a pressure surge, the use of PI or PID control by a conventional PID controller may require many pressure samples and valve position adjustments before the position of the valve is satisfactorily modified. As a result, the actual pipeline pressure may often vary a large amount from the pressure setpoint, in which case the discharge pressure setpoint would have to be set undesirably large amount below the discharge shutdown pressure to avoid repeated shutdown of the system, resulting in reduced flow through the pipeline.